In recent years, nonaqueous electrolyte batteries such as lithium ion secondary batteries have been developed as batteries with high energy density. The nonaqueous electrolyte batteries are anticipated as vehicular power sources such as hybrid vehicles or electric vehicles. In particular, in vehicular use applications, the nonaqueous electrolyte batteries are required to have characteristics such as high-speed charging and discharging performance and long-term reliability. The nonaqueous electrolyte batteries capable of performing high-speed charging and discharging have the advantage that a charging time is considerably short. In hybrid vehicles on which the nonaqueous electrolyte batteries capable of performing high-speed charging and discharging are mounted, power performance can be improved. Moreover, in the hybrid vehicles, regenerative energy of power can be efficiently collected.
The high-speed charging and discharging can be realized by rapid movement of electrons and lithium ions between a positive electrode and a negative electrode. Conventional nonaqueous electrolyte batteries include carbon-based negative electrodes having a negative electrode active material formed of a carbonaceous matter. In nonaqueous electrolyte batteries using carbon-based negative electrodes, dendrites of metal lithium on negative electrodes may precipitate when high-speed charging and discharging are repeated. Dendrites may cause internal short-circuit.
Accordingly, to prevent dendrites of metal lithium from precipitating, batteries including a composite metal oxide as a negative electrode active material instead of a carbonaceous matter have been developed. In particular, batteries using a titanium oxide as a negative electrode active material can perform stable high-speed charging and discharging and have characteristics of a longer lifespan than a carbon-based negative electrode.
However, the potential of a titanium oxide with respect to metal lithium is higher (nobler) than that of a carbonaceous matter. Further, an electric capacity per weight of a titanium oxide is low. Therefore, there is a problem in that a weight energy density is lower in batteries using a titanium oxide.
For example, an electrode potential of a titanium oxide is about 1.5 V based on metal lithium and is higher (nobler) than the potential of a carbon-based negative electrode. Since the potential of a titanium oxide is caused by an oxidation-reduction reaction between Ti3+ and Ti4+ when lithium is electrochemically inserted and desorbed, the potential of the titanium oxide is electrochemically restricted. Also, there is the fact that stable high-speed charging and discharging of lithium ions can be performed in a high electrode potential of about 1.5 V. Thus, it is actually difficult to reduce an electrode potential to improve high energy density per unit weight.
For an electric capacity per unit weight, the theoretical capacity of a lithium-titanium composite oxide such as Li4Ti5O12 is about 175 mAh/g. On the other hand, the theoretical capacity of a general graphite-based electrode material is 372 mAh/g. Accordingly, a capacity density of a titanium oxide is considerably lower than that of a carbon-based negative electrode. This is because the number of sites occluding lithium is small in a crystal structure of the titanium oxide. Also, because lithium is likely to be stabilized in a structure, it is difficult that lithium moves between a positive electrode and a negative electrode, which reduces a substantial electrode potential. That is also another reason.
In view of the above description, new electrode materials containing Ti and Nb have been examined. In particular, a composite oxide represented by TiNb2O has a high theoretical capacity of 387 mAh/g because charge compensations of tetravalent Ti to trivalent Ti and pentavalent Nb to trivalent Nb occur when Li is inserted, and attracts attention.
As a production method of a composite oxide represented by TiNb2O7, the synthesis methods of a solid-phase reaction method, a hydrothermal synthesis method and a sol-gel method are known.
In TiNb2O7, a Li diffusion rate in solid is slow at high SOC (State Of Charge: charging rate of battery), and thus, an overvoltage is large. Thus, the nonaqueous electrolyte battery, which includes TiNb2O7 as a battery active material, has a problem in that charging and discharging cycle performance is poor at high SOC. In order to solve this problem, it is necessary to make a particle size of an active material as small as possible
However, in a conventional solid-phase reaction method, a raw material is solid, and a reaction needs to be carried out by burning-induced elemental diffusion. Thus, burning at a burning temperature of 1000° C. or higher for at least several tens of hours is required in order to obtain a composite oxide prepared by homogeneously mixing Nb and Ti elements. As a result, burned particles become coarse particles, and particle sizes become 1 μm or more. Coarsened particles can be subjected to microparticulation using a mechanical grinding method such as a beads mill, but crystallinity is lowered at the same: time as grinding, and thus, battery characteristics can be deteriorated.
Also, a production method, which uses a sol-gel method using Ti(OC3H7)4 and niobium hydroxide, is known, but it is difficult to form a precursor in which Ti and Nb sources are homogeneously mixed. Thus, burning using high temperature of 1350° C. is carried out, and consequently, coarse particles can be formed.
Also, there is a hydrothermal synthesis method as a method for obtaining microparticles. This hydrothermal synthesis method requires pressurization due to a pressure-resistant closed vessel, corrosion of a closed container can occur during the mass production. Moreover, TiO2 anatase phase can be formed as a heterophase instead of a phase formed of Nb—Ti composite oxide at the time of a hydrothermal treatment, and there are possibilities that uniformity of burned Ti and Nb elements can be deteriorated and that crystallinity is lowered.